1. Field of the Invention
This invention relates to a method of controlling a direct-injection engine incorporating a turbocharger and, more particularly, to a control method for improving fuel combustion during high-load operation to clean exhaust gases, thereby improving fuel economy.
2. Description of Related Art
A turbocharger has been used for increasing an engine torque. Generally, the turbocharger turns a turbine by the use of a high-temperature exhaust gas energy, and turns a compressor which is coupled with the turbine, to thereby compress the air to be taken into the engine. The use of the turbocharger allows the engine to take in a larger quantity of air than the quantity of exhaust gases for the purpose of increasing the engine torque.
On the other hand, in a lean-burn engine typified by the direct-injection engine, a lean air-fuel mixture is burned within the range of low-engine torque operation in order to enhance fuel economy. However, since the quantity of air which is determined by the desired quantity of fuel and the air-fuel ratio is limited by the quantity of engine exhaust gases, the operation range in which the engine is able to operate with a lean mixture is restricted to the range of low engine torque. For example, disclosed in the Japanese Patent Laid-Open No. Hei 11-36867 is a direct-injection engine provided with an intake air quantity increasing means, by which the quantity of intake air is increased when the required quantity of fuel determined by the state of engine operation exceeds a predetermined quantity of fuel.
A recent direct-injection engine is operated at a so-called lean air-fuel ratio for the purpose of enhancing fuel economy; that is, the internal-combustion engine is operated with an excessive-air mixture which is leaner than the mixture of theoretical air-fuel ratio. Furthermore, in the Japanese patent stated above, the supercharger is employed to supply an increased amount of air in an attempt to widen the operation range at the lean air-fuel ratio. The direct-injection engine equipped with such a supercharger operates, in the range of a great engine torque, at a rich air-fuel ratio at which the air-fuel ratio is more on the excess fuel side than the theoretical air-fuel ratio. This operation is performed on the basis of the following phenomenon. Within a high-load range where the engine is developing a great torque, the combustion temperature within the engine combustion chamber rises, likely to produce knocking undesirable to the engine. Usually, to prevent this, a means to lower the combustion temperature by retarding the ignition timing is used. This method, however, allows the exhaust gas temperature to rise. To lower the exhaust gas temperature, much of fuel is injected to cool the exhaust gases with the evaporative latent heat of fuel. Within a high-load range, therefore, the fuel is consumed more than required, resulting in a lowered fuel economy. In the Japanese Patent Laid-Open No. 2000-27427 is described the supercharging, which is effected by the use of a turbocharger to intensify in-cylinder air motion, fully mixing the intake air with much of fuel injected, and accordingly producing a good homogenous combustion. The above-described prior invention, however, is still not enough for improving fuel economy within a range of high load.
Therefore, in a low-load operation range of engine torque, lean-burn operation is able to enhance fuel economy. In a high-load operation range, however, there arises a lowering of fuel economy, on the whole, fuel economy is not improved under actual driving condition.
Furthermore, when the lean-burn air-fuel ratio has increased in the direct-injection engine incorporating the supercharger, there takes place such a problem that, within a comparatively great-load range of the lean-burn range, the fuel concentrates excessively in one part within the combustion chamber, causing smoke to occur.